Use of a polyster composition as a hydraulic fluid

ABSTRACT

The present invention relates to the use of polyesters which consist of at least one carbohydrate and at least one carboxylic acid, and mixtures thereof, as a hydraulic fluid.

The present invention relates to the use of polyesters which consist ofat least one carbohydrate and at least one carboxylic acid, and mixturesthereof, as a hydraulic liquid or hydraulic fluid.

Hydraulic drives enable the transfer of large forces using relativelysimple elements and the stepless variation of working speeds, andhydraulic energy transport take place between the generating section,the pumps and the motor section, the hydraulic motors or turbines, via ahydraulic fluid. Depending on the mode of action, a distinction is drawnbetween hydrostatic and the less common hydrokinetic drives. Thehydrostatic drive works by the principle of volume displacement inenclosed, changing spaces. The pump displaces a certain volume ofhydraulic fluid per stroke or revolution against a pressure existing inthe flow (hydraulic motor). The hydrokinetic drive functions by theprinciple of inertia by deflecting a mobile hydraulic fluid mass inblade cascades arranged in rotational symmetry (turbines).

In the fulfillment of the function of hydraulic systems, the hydraulicfluid is of substantial importance. The fundamental objects of thehydraulic fluid are the transfer of output or signals to hydraulicsystems. Hydraulic oils or hydraulic fluids therefore refer to thoseliquid substances or mixtures which are suitable for energy transfer inhydrostatic or hydrokinetic (hydrodynamic) systems. In addition toenergy transfer, hydraulic fluids should ensure sufficient lubricationof lubrication points and the protection of the components of thehydraulic drive from corrosion, and also remove heat from the system.Irrespective of the different conditions in different hydraulic systems,the hydraulic pressure medium always has to guarantee trouble-freefunctioning of the individual hydraulic components. In addition toappropriate flow behavior and good compressibility, i.e. low volume andpressure variation under pressure, for the disruption-free transport ofenergy, hydraulic fluids therefore have to have very good slidingproperties for lubrication, high specific heat for cooling, goodcompatibility with the system materials and corrosion protectionproperties.

The different hydraulic machines and units have highly differingoperating conditions, for example extremely high or extremely lowtemperatures. Depending on the application, the hydraulic fluidstherefore have to have not only the aforementioned general functionalproperties, but also application-specific properties which may be verydifferent from case to case. For example, hydraulic fluids for hydraulicsystems of airplanes have to have particularly good low temperatureproperties. In fire-endangered hydraulic units, for example inbituminous coal mining, low-flammability hydraulic fluids in particularare used. The brake fluids which likewise belong to the hydraulic fluidshave to be, for example, cold-, heat- and aging- and alsooxidation-resistant, noncorrosive and not have any effect on rubber.

The specific characteristic data of hydraulic liquids which are ofinterest especially from an application point of view includeviscosity-temperature behavior, viscosity-pressure behavior and thedetermination of the density-temperature dependence. The change in theviscosity with the pressure or the temperature is well known for manyconventional hydraulic fluids, in particular from the group of themineral oils. With increasing temperature, for example, mineral oilsused as a hydraulic liquid have distinctly lower viscosities than atlower temperatures. When the viscosity goes below a lower minimum as aresult of excessively high temperatures, this results in mixed or solidstate friction in hydraulic components, which increases the overallfriction and intensifies the wear. In contrast, excessively highviscosities are to be avoided in particular from an energetic point ofview.

Like other hydraulic components, the hydraulic medium also undergoesaging during the time, which manifests itself in a change in thecharacteristic physical and chemical parameters. For example, thehydraulic medium loses its good tribological properties, and itsoriginal object, namely the protection of the components from corrosion,becomes reversed, which may result in a corrosive attack on thecomponents of the hydraulic drive.

Every year, about 160 000 metric tons of hydraulic fluid products areused in Germany, about 40% of the total amount of hydraulic fluid beingaccounted for by mobile applications and 60% by stationary applications.These are in particular the hydraulic oils H (aging-resistant withoutactive additives), HL (with active ingredients for increasing the agingresistance and the corrosion protection), HLP (additionally with activeingredients for preventing wear in the mixed friction range) and HVLP(additionally -with active ingredients for improving theviscosity-temperature behavior). The hydraulic fluid products alsoinclude the low-flammability hydraulic fluids which are used inparticular in fire-endangered hydraulic units and for which adistinction is drawn between the types HFA (oil-in-water emulsions), HFB(water-in-oil emulsions), HFC (aqueous polymer solutions, for example ofpolyglycols) and HFD (anhydrous fluids, for example phosphoric esters,silicic esters, silicones, halohydrocarbons and others). Likewiseincluded are brake fluids which, in hydraulic brake systems of vehicles,serve to transmit the brake pressure and consist conventionally ofglycols, glycol ethers and/or polyalkyl glycols.

According to an inquiry of the German Federal Environmental Office(1997), 73.2% of the hydraulic fluids used in the industrial sector arecollected again after use and fed to recycling. This means that about30% of all hydraulic fluids remain in the environment. A large portionof these products gets into the environment, into soils or groundwaterand surface water as a result in particular of leaks from hydraulicsystems or as a result of the loss of drops, for example when hydraulichoses are changed in excavators. However, as a consequence of theirchemical composition, many hydraulic fluids are hardly decomposed, orare decomposed only very slowly, by the natural systems of theenvironment, for example microorganisms. This constant contamination ofthe soil, the groundwater and surface water with hydraulic fluidstherefore leads in the long term to considerable risks for flora, faunaand humans.

In view of the risks to the environment which arise in particular fromhydraulic fluids based on mineral oils, efforts have been made for sometime to develop biodegradable hydraulic oils. For example, hydraulicoils have been developed which are based on vegetable oils such asrapeseed oil or sunflower oil and derivatives thereof. Although theseproducts based on natural, in particular vegetable, oils offer theadvantage of relatively rapid biodegradability, they frequently do nothave, or have only to an insufficient extent, the properties requiredfor use as a hydraulic fluid, such as viscosity-temperature behavior,long-term cold stability, aging stability, etc.

Further renewable raw materials such as sugar and starch have hithertoremained unused for the application as a hydraulic fluid and theirpotential as a polyol constituent for synthetic esters is virtuallyunresearched. However, their availability makes such raw materials veryattractive, especially because their natural origin gives rise toadvantages with regard to rapid biodegradability and environmentalcompatibility. The prior art merely discloses a few applications ofsugar compounds in the lubricants field, but not in the hydraulic fluidsfield.

EP 0 879 872 A1 discloses biodegradable, nontoxic lubricant oilformulations which consist of an ester of a sugar and of a fatty acid.The polyol constituent of the polyester may include a sugar, sugaralcohol or a mixture thereof. The nontoxic lubricant oil formulationdescribed is intended to find use in particular in aggregates which areused in the agricultural and food industry, or in the cosmetics orpharmaceutical industry.

EP 0 572 198 A1 describes lubricant compositions which may likewise beused for machines for producing foods. The compositions comprise amixture of a first ester of a medium-length saturated fatty acid withglycerol (component A) and of a second ester of a carboxylic acid withsucrose.

DE 42 29 383 C2 describes an edible lubricant with the addition oflubricity-improving esters of fatty acids and higher alcohols. Theadditives for improving the lubricity consist of at least two esters ofedible alcohols having at least two alcohol groups. Useful alcoholsinclude, for example, glycerol, pentaerythritol, arabitol, mannitol andsorbitol.

The technical problem on which the present invention is based is thus toprovide synthetic esters whose structure is based fully on renewable rawmaterials and which are obtainable in particular using low molecularweight sugars and from fatty acids which can be isolated from vegetablesources as base fluids for hydraulic fluids, these synthetic esters onthe one hand having the required performance properties such asoxidation and aging stability, thermal stability, suitableviscosity-temperature behavior, suitable viscosity-pressure behavior,etc., and, on the other hand, as a consequence of their natural origin,being rapidly biodegradable and thus environmentally compatible to ahigh degree.

The invention solves the technical object on which it is based by theuse of a rapidly biodegradable composition comprising at least onepolyester or at least one polyester derivative and a polyester mixture,the polyester having been formed from a carbohydrate, each of which hasbeen esterified with at least one carboxylic acid, at least onecarboxylic acid derivative or a mixture thereof, as a hydraulic oil.

The polyester or sugar ester present in the composition is obtained bychemically joining the carbohydrate and the carboxylic acid or thecarboxylic acid derivative or a mixture thereof. Most sugar estersbelong to the class of the nonionic surfactants. Owing to theiramphiphilic character, their particular readiness to biodegrade andtheir good surface properties, these sugar esters have hitherto beenused mainly in the foods industry, cosmetics and pharmaceuticals. Thepresent invention thus provides for the first time the use of such sugaresters as the base fluid of hydraulic oils. The invention provides inparticular that the polyester or sugar ester has a structure based fullyon renewable raw materials, in particular indigenous raw materials, forexample vegetable oils and fats. In other words, all constituents whichare used to synthesize the polyester used as a hydraulic fluid inaccordance with the invention, i.e. both carbohydrate constituent andcarboxylic acid constituent, are obtained from renewable, in particularvegetable, raw materials, for example vegetable oils and fats. Thepolyester having such a structure may then, when it gets into theenvironment, for example into soils or surface water or groundwater, berapidly degraded by natural systems such as microorganisms as aconsequence of its natural constituents.

Investigations of the applicant of the present invention have shown thatthe polyesters used in accordance with the invention have outstandinghydraulic fluid properties, for example viscosity behavior which isexquisitely suitable for this field of application, load-bearingcapability, wear behavior, very good air release capability and verygood oxidative aging stability.

In connection with the present application, the term “use as a hydraulicoil”, “use as a hydraulic fluid” or “use as a hydraulic liquid” meansthat a substance or substance mixture which is either liquid by natureor in its liquid form after dissolution in a liquid medium hasproperties which enable use of the substance in hydrostatic orhydrokinetic (hydrodynamic) systems for energy transfer. According tothe invention, “use as a hydraulic fluid” means in particular the use asa base fluid for hydraulic oils and does not rule out the addition offurther conventionally used additives for hydraulic oils, such asphenolic and/or aminic antioxidants, phosphorus/sulfur extremepressure/antiwear additives, corrosion inhibitors, foam inhibitors andother performance-improving additives.

A preferred embodiment of the invention therefore provides for the useof a carbohydrate ester composition comprising at least one polyester orat least one polyester derivative or a mixture thereof, the polyesterconsisting of a carbohydrate and at least one carboxylic acid, aderivative thereof or a mixture thereof as a base fluid for hydraulicoils, the composition additionally comprising additives which aretypical for hydraulic oil and are selected from the group consisting ofantioxidants, high-pressure and wear additives, corrosion inhibitors,foam inhibitors and viscosity regulators.

In connection with the present invention, “rapidly biodegradable” meansthat the polyester composition used in accordance with the invention israpidly degraded by the biological systems of the environment, inparticular microorganisms such as bacteria and fungi which are presentin the environment. The resulting low molecular weight degradationproducts also do not constitute any environmental pollution, eitherbecause they are already naturally occurring substances which arenontoxic for flora and fauna and are thus environmentally compatible, orbecause the decomposition products may be degraded by successivebiological systems, in particular further microorganisms, to suchnaturally occurring nontoxic substances. The present invention thusprovides that the natural degradation of the composition used as ahydraulic fluid in accordance with the invention leads substantially toproducts which are safe for organisms such as animals and humans.

In connection with the invention, “at least one polyester” means thatthe composition used in accordance with the invention as a base fluidfor hydraulic oils contains at least one polyester, but may contain aplurality of different polyesters. “At least one carbohydrate” meansthat the different polyesters present in the composition contain atleast one carbohydrate radical, and this carbohydrate may be esterifiedwith a carboxylic acid, but also with different carboxylic acids.However, the polyesters used in accordance with the invention may alsoinclude a plurality of different carbohydrate constituents which may beesterified either with only one carboxylic acid or one carboxylic acidderivative or with different carboxylic acids and/or carboxylic acidderivatives. “At least one carboxylic acid or at least one derivativethereof” means that a carbohydrate radical present in the compositionmay be esterified with at least one carboxylic acid radical or at leastone derivative of a carboxylic acid, or else with different carboxylicacids or different carboxylic acid derivatives or a mixture thereof.

The present invention provides in particular that the carbohydrateconstituent of the polyester composition used in accordance with theinvention may be isolated directly from renewable raw materials, inparticular indigenous vegetable raw materials, or may be preparedinexpensively from natural products in few industrial steps. In apreferred embodiment of the invention, the carbohydrate is amonosaccharide, disaccharide, trisaccharide, a sugar alcohol derivedtherefrom, a starch hydrolyzate, fructooligosaccharides, a hydrogenatedproduct thereof, a mixture thereof or dehydrated intermediates of thecarbohydrate, for example sorbitan, dianhydrosorbitol, etc.

In a preferred embodiment of the invention, the carbohydrate is xylose,arabinose, ribose, maltose, lactose, sucrose, raffinose, glucose,mannose, galactose, sorbose, fructose, isomaltulose, trehalulose,lactitol, maltitol, hydrogenated maltotriose, sorbitan, xylitol,sorbitol, mannitol, erythritol, arabitol,6-O-α-D-glucopyranosyl-D-sorbitol (1,6-GPS),1-O-α-D-glucopyranosyl-D-sorbitol (1,1-GPS),1-O-α-D-glucopyranosyl-D-mannitol (1,1-GPM), isomalt, or a mixturethereof. In a particularly preferred embodiment of the invention, thesugar alcohol used as a starting substance for preparing the polyesterused in accordance with the invention is sorbitol.

A further preferred embodiment of the invention provides that thepolyester contains, as the acid constituent, an unbranched and/orbranched saturated or unsaturated monocarboxylic acid, dicarboxylicacid, tricarboxylic acid, a derivative thereof or an isomer thereof,each of which may be directly isolated from renewable raw materials, inparticular indigenous vegetable raw materials, for example vegetableoils and fats, or may be prepared from natural products inexpensively infew industrial steps and is therefore particularly efficientlybiodegradable.

The chain length of the acid constituent has a significant influence onthe properties of the polyester, for example viscosity-temperaturebehavior, viscosity-pressure behavior and material compatibility. Thepresent invention therefore provides that the carbohydrate constituentof the composition used in accordance with the invention has beenesterified in particular with monocarboxylic acid, preferably aC₂-C₂₄-monocarboxylic acid, more preferably a C₄-C₁₈-monocarboxylicacid.

A particularly preferred embodiment of the invention therefore relatesto the use of a composition in which the carbohydrate has beenesterified with acetic acid, butyric acid, isobutanoic acid, valericacid, isovaleric acid, caproic acid, enanthic acid, caprylic acid,2-ethylcaproic acid, pelargonic acid, capric acid, isostearic acid,lauric acid, myristic acid, myristoleic acid, palmitic acid, palmitoleicacid, stearic acid, oleic acid, elaidic acid, ricinoleic acid, linoleicacid, linolenic acid, eleostearic acid, arachic acid, behenic acid orerucic acid or a mixture of these acids.

The present invention also provides that the polyester which is to beused as a hydraulic oil in accordance with the invention also contains,as the acid component, a derivative of a carboxylic acid such as ananhydride, mixed anhydride, an alkyl ester or a carbonyl chloride.Anhydrides are the products of an acid, for example a carboxylic acid,which are obtainable, for example, by dehydration. The loss of waterfrom two different acids may result in mixed anhydrides. Alkyl estersmay be prepared by an acid-catalyzed reaction of carboxylic acids withalcohols.

A further preferred embodiment of the invention therefore relates tocompositions in which the open-chain and cyclic D-sorbitol andD-mannitol derivates have been esterified with carboxylic acidderivatives, for example anhydrides, mixed anhydrides, alkyl esters, inparticular carbonyl chlorides.

In a further preferred embodiment, the sugar alcohol derivatives mayalso have been esterified with isomers of carboxylic acids such ascis/trans isomers within the structure or at geometric positions.Isomers are compounds having the same empirical, but differentstructural, formulae. Cis/trans isomers are stereoisomers which featurea different atom arrangement in three-dimensional space, in particular adifferent arrangement of the substituents. Stereoisomers thereforediffer in the configuration and/or the confirmation. A particularlypreferred embodiment of the invention therefore relates to the use of acomposition in which the carbohydrate has been esterified with aderivative or an isomer of monocarboxylic acid.

A preferred embodiment of the invention provides that the carbohydrateester intended for use as a hydraulic fluid has such a degree ofesterification that at least 75% of all free hydroxyl groups of thecarbohydrate have been esterified. A particularly preferred embodimentof the invention provides that all free hydroxyl groups of thecarbohydrate have been esterified with at least one carboxylic acid, atleast one carboxylic acid derivative or a mixture thereof.

A further embodiment of the invention relates to the use of acarbohydrate ester as a hydraulic oil, wherein the polyester can beprepared by esterifying or transesterifying the carbohydrate or amixture comprising a plurality of different carbohydrates, in solventsor without solvent, in the presence of a catalyst. The products whichare intended for use as a hydraulic oil or hydraulic fluid may thus beprepared in known organic solvents such as toluene, DMSO, pyridine, DMFand the like, but also without solvent, by esterifying ortransesterifying the carbohydrate (polyol) mixture with the appropriatereagents with the addition of suitable catalysts.

The present invention provides in particular that the products used ashydraulic fluids in accordance with the invention are prepared usingtransition metal compounds of in particular Sn, Ti or Zn/Cu, for examplesalts, oxides, alkyls, etc., mineral acids such as HCl, H₂SO₄ or H₃PO₄,organic acids such as p-toluenesulfonic acid, methanesulfonic acid orsulfosuccinic acid, acidic ion exchangers, alkaline metal salts such ashydroxides, carbonates, methoxides, ethoxides of, for example, sodium orpotassium, zeolites or mixtures thereof.

Particular preference is given in accordance with the invention to usingp-toluenesulfonic acid or a tin oxalate catalyst as the catalyst.

The present invention provides that the carbohydrate esters intended foruse as a hydraulic fluid are prepared by transesterifying or esterifyingin one or more solvents or without solvent. Particular preference isgiven in accordance with the invention to organic solvents such astoluene, DMSO, pyridine or DMF.

The present invention provides that the carbohydrate esters intended foruse as a hydraulic fluid are prepared by a transesterification oresterification at a temperature of from 120° C. to 280° C. Particularpreference is given to using carbohydrate esters which are prepared atan esterification or transesterification temperature of from 160° C. to250° C.

The ratio of the carbohydrate and acid starting constituents, inparticular the ratio of hydroxyl groups to carboxylic acid groups, inthe esterification or transesterification has a decisive influence onthe degree of esterification of the resulting carbohydrate esters. Theamount of the acid constituents to be used in the reaction, based on theamount of the carbohydrate constituent used, depends upon how many freehydroxyl groups the carbohydrate used as a starting material has. Thepresent invention provides in particular that the ratio of hydroxylgroups to carboxylic acid groups is from 1:1 to 1:10. A preferredembodiment of the present invention therefore relates to the use ofcarbohydrate esters in whose preparation the ratio of hydroxyl groups tocarboxylic acid groups of the starting constituents is from 1:1 to 1:10.Particular preference is given to using carbohydrate esters in whosepreparation the starting ratio of hydroxyl groups to carboxylic acidsgroups of the carbohydrate and acid starting constituents is from 1:1.5to 1:7.

The reaction time used for the esterification or transesterificationalso has a decisive influence on the degree of esterification of theresulting products. The present invention provides that the duration ofthe esterification or transesterification of the carbohydrates withcarboxylic acids is from 2 to 36 hours, more preferably from 4 to 26hours, most preferably from 8 to 10 hours. A preferred embodiment of thepresent invention thus relates to the use of carbohydrate esters whosepreparation by esterifying or transesterifying a carbohydrate with acarboxylic acid or a carboxylic acid derivative takes from 2 to 36hours, more preferably from 4 to 26 hours, most preferably from 8 to 10hours.

The inventively preferred reaction conditions for esterifying ortransesterifying carbohydrates to prepare the polyesters used inaccordance with the invention preferably include the followingparameters: use of a stirred reactor, although the esterification mayalso be carried out in accordance with the invention in from 2 to 5stages in a stirred tank battery, removal of water during the reactionby rectification or azeotropic rectification, carrying out the reactionin an organic solvent, for example toluene, DMF or ether, or withoutsolvent, a reaction time of from 2 to 36 hours, preferably from 8 to 26hours, and carrying out the reaction in the presence of a catalyst, theamount of catalyst based on the total amount being 0.05-10% by weight,preferably 0.1-5% by weight. The carbohydrate and acid startingsubstances, based on the monomer units, are preferably in a ratio offrom 1:1 to 1:10, more preferably in a ratio of from 1:1.5 to 1:7. Thereaction is preferably effected under a reduced pressure of from 300 to10 mbar.

The carbohydrate esters intended in accordance with the invention foruse as hydraulic fluids or hydraulic oils have outstandingphysicochemical properties which predestine them in particular for thisfield of application. The carbohydrate esters used in accordance withthe invention have, for example, a kinematic viscosity of 40° C. of fromabout 20 to 120 mm²/s. Investigations have also shown that they have anexcellent long-term cold stability, since they are still free-flowingafter several days at −25° C. Their outstanding oxidative agingstability can be demonstrated in the turbine oil stability test.

A preferred embodiment of the invention therefore relates to the use ofcarbohydrate ester compositions which have a kinematic viscosity of from20 to 120 mm²/s as hydraulic fluids. A further embodiment of theinvention relates to the use, as hydraulic fluids, of carbohydrateesters which have such a long-term cold stability that they are stillfree-flowing after three days at a temperature of −25° C. Yet anotherpreferred embodiment of the invention relates to the use, as a basefluid for hydraulic oils, of a carbohydrate ester composition which hasa pour point lower than −25° C. The present invention also provides thatthe carbohydrate ester compositions which are used as hydraulic fluidshave a load-bearing capability of at least load stage 10 in the FZGA/8.3/90 test method. The present invention further provides thatcarbohydrate ester compositions are used, as hydraulic fluids, whichrequire more than 1 800 hours in the turbine oil stability test withoutthe addition of water until an acid number of 2 mg KOH/g has beenattained.

The present invention is illustrated in detail by the examples whichfollow.

EXAMPLE 1

Preparation of a sugar ester by esterifying D-sorbitol and D-mannitolwith caprylic anhydride (batchwise variant)

In a stirred reactor, 250 g of a 1:1 mixture of D-sorbitol andD-mannitol were dehydrated at 155° C. for 1.25 hours in the presence of0.8 g of p-toluenesulfonic acid. After 1.86 kg of caprylic anhydride and6 g of tin oxalate had been added, the mixture was stirred at 195° C.for 10 hours, in the course of which water was removed by distillation.On completion of reaction and removal of the catalyst, the excess acidwas removed under reduced pressure. The product obtained was a clearbright yellow oil.

EXAMPLE 2

Use of the product fully esterified with caprylic anhydride as ahydraulic fluid

The product obtained in example 1 from the esterification reaction ofD-sorbitol and D-mannitol with caprylic anhydride (n-C8) was tested as abase fluid for hydraulic oils. The product obtained in example 1 wasadditized with additives typical of hydraulic oils, such as phenolic andaminic antioxidants, phosphorus/sulfur extreme pressure/antiwearadditives, corrosion inhibitors and a foam inhibitor. Subsequently, theproperties of this mixture were investigated with a view to itssuitability as a hydraulic fluid. The following results were obtained:

Kinematic viscosity at 40° C.: 36 mm²/s

Pour point: −30° C. The measurement was in accordance with DIN ISO 3016.This value is to be regarded as good.

Long-term cold stability: still free-flowing after 3 days at −25° C.This value is to be regarded as good.

Air release capability at 50° C.: 3 minutes. The measurement was inaccordance with DIN 51381. This value is to be regarded as good.

Demulsification capability: 25 minutes at 50° C. The measurement was inaccordance with DIN 51599. This value is to be regarded as good.

Load-bearing capability/wear behavior: still free of damage at loadstage 11 in the FZG A/8.3/90 test method. The wear scar diameter was0.31 mm in a four-ball apparatus to DIN 51350. The values are to beregarded as very good.

Aging stability: 1900 hours in the turbine oil stability test withoutthe addition of water until an acid number of 2 mg KOH/g has beenattained.

COMPARATIVE EXAMPLE 1

Testing of fully esterified glycerol for suitability as a hydraulicfluid

The base fluid used was glycerol which had been fully esterified with amixture of caprylic acid and caproic acid. The resulting product wasadditized with phenolic and aminic antioxidants, phosphorus/sulfurextreme pressure/antiwear additives, corrosion inhibitors and a foaminhibitor, and the additives were identical to the additives in example2. The following properties were determined for this base fluid:

Kinematic viscosity at 40° C.: 15 mm²/s. For most applications, thisvalue is too low.

Pour point: −10° C. The measurement was in accordance with DIN ISS 3016.This value is not low enough for most applications, especially in colderclimates.

Long-term cold stability: after 3 days at −25° C., no longerfree-flowing. This value is unacceptable for applications in coldclimates.

Air release capability at 50° C.: 6 minutes. The measurement was inaccordance with DIN 51381. The value is to be regarded as moderatelygood.

Demulsification capability: 20 minutes at 50° C. The measurement was inaccordance with DIN 51599. The value is to be regarded as good.

Load-bearing capability/wear behavior: still free of damage at loadstage 10 in the FZG A/8.3/90 test method. Wear scar diameter 0.35 mm ina four-ball apparatus to DIN 51350. Both values are to be regarded asmoderately good.

Aging stability: 1200 hours in the turbine oil stability test withoutthe addition of water until an acid number of 2 mm KOH/g has beenattained. This value is to be regarded as moderately good.

COMPARATIVE EXAMPLE 2

Testing of glycerol which had been esterified with sunflower oil fattyacid with a view to suitability as a hydraulic fluid

The base fluid used was glycerol which had been fully esterified withsunflower oil fatty acid (high oleic quality, oleic acid fraction 80%).The glycerol ester was additized with phenolic and aminic antioxidants,phosphorus/sulfur extreme pressure/antiwear additives, corrosioninhibitors and a foam inhibitor. The additives were identical to thoseused in example 2. For the base fluid obtained in this way, thefollowing properties were determined:

Kinematic viscosity at 40° C.: 38 mm²/s

Pour point: −10° C. The measurement was in accordance with DIN ISO 3016.This value is not low enough for most applications, especially in colderclimates.

Long-term cold stability: After three days at −25° C., no longerfree-flowing. This value is unacceptable for use in cold climates.

Air release capability at 50° C.: 4 minutes. The measurement was inaccordance with DIN 51381. The value is to be regarded as good.

Demulsification capability: 22 minutes at 50° C. The measurement was inaccordance with DIN 51599. The value is to be regarded as good.

Load-bearing capability/wear behavior: still without damage at highestload stage (12) in the FZG A/8.3/90 test method. Wear scar diameter 0.31mm in the four-ball apparatus to DIN 51350. Both values are to beregarded as very good.

Aging stability: 450 hours in the turbine oil stability test without theaddition of water until an acid number of 2 mm KOH/g has been attained.This value is to be regarded as poor.

1. A method for making a base fluid for a rapidly biodegradablehydraulic fluid, said base fluid having low cold viscosity, wherein themethod comprises the steps of: (a) forming a carbohydrate mixturecomprised of mannitol and sorbitol; and (b) esterifying saidcarbohydrate mixture with at least one selected from the groupconsisting of a C₂ to C₂₄ monocarboxylic acid, a C₂ to C ₂₄monocarboxylic acid derivative and mixtures thereof to convert saidcarbohydrate mixture to a polyester mixture, such that at least 75% ofall free hydroxyl groups in the carbohydrate mixture are esterified,wherein the polyester mixture is said base fluid.
 2. The method asclaimed in claim 1, wherein the carbohydrate mixture is esterified witha C₄ to C₁₈ monocarboxylic acid.
 3. The method as claimed in claim 1,wherein the carbohydrate mixture is esterified with the C₂ to C₂₄monocarboxylic acid and wherein the monocarboxylic acid is selected fromthe group consisting of acetic acid, butyric acid, isobutanoic acid,valeric acid, isovaleric acid, caproic acid, enanthic acid, caprylicacid, 2-ethylcaproic acid, pelargonic acid, capric acid, isostearicacid, lauric acid, myristic acid, myristoleic acid, palmitic acid,palmitoleic acid, stearic acid, oleic acid, elaidic acid, ricinoleicacid, linoleic acid, linolenic acid, eleostearic acid, arachic acid,behenic acid, erucic acid and mixtures thereof.
 4. The method as claimedin claim 1, wherein the carbohydrate mixture is esterified with saidmonocarboxylic acid derivative, and wherein the derivative is ananhydride, a mixed anhydride, an alkyl ester or a carbonyl chloride. 5.The method as claimed in claim 1, wherein substantially all freehydroxyl groups in the carbohydrate mixture are esterified.
 6. Themethod as claimed in claim 1, wherein the carbohydrate mixture isesterified in the presence of a catalyst.
 7. The method as claimed inclaim 6, wherein the catalyst is one selected from the group consistingof a transition metal compound, a mineral acid, an organic acid, anacidic ion exchanger, an alkali metal salt, a zeolite and mixturesthereof.
 8. The method as claimed in claim 7, wherein the catalyst isthe transition metal compound and wherein the transition metal compoundis selected from among salts, oxides and alkyls of Sn, Ti and Zn/Cu. 9.The method as claimed in claim 7, wherein the catalyst is the mineralacid and wherein the mineral acid is selected from among HCl, H₂SO₄ andH₃PO₄.
 10. The method as claimed in claim 7, wherein the catalyst is theorganic acid and wherein the organic acid is selected from amongp-toluenesulfonic acid, methanesulfonic acid and sulfosuccinic acid. 11.The method as claimed in claim 7, wherein the catalyst is the alkalimetal salt and wherein the alkali metal salt is selected from amonghydroxides, carbonates, methoxides and ethoxides of sodium and ofpotassium.
 12. The method as claimed in claim 7, wherein the catalyst isp-toluenesulfonic acid or a tin oxalate catalyst.
 13. The method asclaimed in claim 1, wherein the esterification is effected in at leastone solvent.
 14. The method as claimed in claim 13, wherein the solventis an organic solvent.
 15. The method as claimed in claim 14, whereinthe solvent is selected from among toluene, DMSO, pyridine and DMF. 16.The method as claimed in claim 1, wherein the esterification occurs at atemperature of from 120° C. to 280° C.
 17. The method as claimed inclaim 1, wherein a ratio of hydroxyl groups to carboxylic acid groups inthe esterification reaction is from 1:1 to 1:10.
 18. The method asclaimed in claim 1, wherein the reaction time for the esterificationreaction is from 2 to 36 hours.
 19. The method as claimed in claim 1,wherein the base fluid has a kinematic viscosity at 40° C. of from 20 to120 mm²/s.
 20. The method as claimed in claim 1, wherein the base fluidhas a long-term cold stability that renders it still free flowing after3 days at −25° C.
 21. The method as claimed in claim 1, wherein the pourpoint of the base fluid is less than −25° C.
 22. The method as claimedin claim 1, wherein the base fluid has a load bearing capability of atleast load stage 10, determined in accordance with the FZG A/8.3/90 testmethod.
 23. The method as claimed in claim 1, wherein the base fluid hasan aging resistance such that more than 1800 hours are required in theturbine oil stability test without the addition of water to attain anacid number of 2 mg KOH/g.